Well tree isolation relates to the isolation of wellhead equipment on a hydrocarbon well from the high pressures and/or abrasive fluids required for well stimulation. A proportion of all oil and gas wells eventually require some form of stimulation to enhance their hydrocarbon flow and make them economically viable. Stimulation of an oil or gas well may be accomplished by several methods. One method involves pumping an acidic solution under pressure into the hydrocarbon bearing formation of a well. Another method is to hydraulically fracture the hydrocarbon bearing formations of a well. Hydraulic fracturing is the process of breaking open a hydrocarbon bearing formation by forcing into it liquids and/or gases which may be laden with abrasives. Extremely high pressures and high flow rates must be employed in the hydraulic fracturing process.
Wellhead equipment includes gate valves, ball valves, blow-out prevention stacks, drilling spools, tubing bonnets, tubing spools, casing spools, casing bores and all related flanges in various combinations, collectively referred to as a well tree. Generally, the well tree provides a means for safely controlling the flow from an oil, gas or water well which occurs from a hydrocarbon or water-bearing formation, the product being brought to the surface by means of production tubing strings. However, the conventional well trees installed in most oil and gas wells are generally not designed to withstand the pressures required to hydraulically fracture a well or, in some cases, to inject caustic fluids into the well. Most conventional well trees are designed for pressures of 21,000 kpa or less while pressures in excess of 21,000 kpa are often required in the hydraulic fracturing process. Therefore, when a well is to be stimulated, the well tree must either be upgraded to the necessary pressure requirements or it must be isolated from the elevated pressures required for the well stimulation process.
There are several known methods of isolating well trees. All the known methods are alike in that they require the insertion of a length of a high pressure tubing through a vertical passage defined by the well tree valves and flanges, the lower end of the high pressure tubing being sealed or packed-off in the production tubing or casing. Each method also requires a sealing mechanism attached to the top of the well tree to prevent well bore pressure from escaping into the atmosphere during insertion or the removal of high pressure tubing, and each requires a high pressure valve affixed to the top of the high pressure tubing to control pressure while the high pressure tubing is seated and packed-off in the production tubing or casing. The above principles of well tree isolation are common to all well tree isolation equipment. The difference in the well tree isolation methods reside in the mechanism by which the high pressure tubing is inserted through the well tree.
The Applicant's U.S. Pat. No. 4,867,243, for example, entitled WELLHEAD ISOLATION TOOL AND SETTING DEVICE AND METHOD OF USING SAME which issued on Sep. 19, 1989 discloses a well tree isolation apparatus. The apparatus comprises a single hydraulic cylinder supported in an axial alignment over a well tree by at least two elongated support rods. The hydraulic cylinder support rods are connected between a base plate and a hydraulic cylinder mounting plate to support the hydraulic cylinder above the well tree at a distance approximately equal to the height of the well tree. The apparatus permits the insertion of a single length of high pressure tubing through any well tree, regardless of its height. Once the high pressure tubing is seated in a production tubing or well casing, the hydraulic cylinder, hydraulic cylinder plate and supports rods are removed to provide 360.degree. access to a high pressure valve attached to the top of the high pressure tubing.
Nevertheless, U.S. Pat. No. 4,867,243 fails to address an application of the apparatus disclosed therein to a dual string well tree. The dual string well tree includes two vertical passages, defined by two separate sets of well tree valves and flanges for respective control of fluid flow from two production tubing strings. The two production tubing strings extend through a single well casing and usually communicate with different hydrocarbon bearing formations. The dual string completion is less expensive and quicker to install than the more conventional use of a workover rig and workover fluids. For example, U.S. Pat. No. 5,775,420 entitled DUAL STRING ASSEMBLY FOR GAS WELLS, which issued to Mitchell et al. on Jul. 7, 1998, describes a dual completion for gas wells which includes a dual base with a primary hanger incorporated in the base. Primary and secondary coiled tubing strings extend through the base at a downwardly converging angle of two degrees or less. The dual base is mounted on an annular blowout preventer. At the top of the annular blowout preventer is a tubing centralizer that aligns the two tubing strings parallel to one another. The blowout preventer has two side ports below the bladder, permitting an operator to produce gas from the annulus, to flare gas to atmosphere or to pump in kill fluid in the event of an emergency. The alignment of the tubing strings allows production recorders to be run in either string.
The difficulty in isolating the dual string well trees is the closeness of the two well trees and the consequent lack of working space to enable a prior art well tree saver to be mounted above each well tree. To date, there has been no solution proposed for this problem.